The present invention relates to a curable fluorosilicone composition or, more particularly, to a fluorosilicone composition having good curability and useful as a molding compound for injection molding, gel-like potting material or adhesive or capable of giving a cured silicone rubber having excellent resistance against oils and organic solvents, restorability from compression and mechanical strengths so as to be useful as a material for shaping diaphragms and oil seals as a part of, for example, equipments for transportation.
As is well known, fluorosilicone rubbers or, namely, silicone rubbers of which the organopolysiloxane molecules have siliconbonded hydrocarbon groups at least partly substituted by fluorine atoms for the hydrogen atoms, such as 3,3,3-trifluoropropyl group, have excellent characteristics such as heat resistance, cold resistance, resistance against oils and solvents, restorability from compression and so on in comparison with other conventional nonfluorine silicone rubbers so that they are widely used as a material for shaping various parts of transportation equipments such as automobiles and aircrafts and various parts of machines used in petroleum industry. See, for example, U.S. Pat. Nos. 2,979,519 and 3,179,610 teaching a silicone rubber composition of which the organopolysiloxane molecules have perfluorinated hydrocarbon groups and which are capable of giving cured silicone rubbers having excellent resistance against hydrocarbon solvents. These prior art fluorosilicone rubber compositions are, however, defective in respect of the low curability that the composition cannot be cured at a relatively low temperature such as room temperature and in respect of the poor mechanical properties of the rubber products obtained by curing the composition even when the composition has been fully cured.
It is usual that organopolysiloxane gums as a principal ingredient of a silicone rubber composition are prepared by the ringopening polymerization of a cyclic diorganosiloxane oligomer using an alkali catalyst in the presence of a chain terminal-forming agent such as a hexaorganodisiloxane. It is sometimes desirable to use 1,1,3,3-tetramethyl-1,3-divinyl disiloxane as the chain terminal-forming agent so that a vinyl group is introduced into each terminal of the diorganopolysiloxane molecules to serve as a crosslinking point. When a fluorine-substituted cyclic diorganosiloxane oligomer is subjected to the ring-opening polymerization in the above described manner using an alkali catalyst in the presence of 1,1,3,3-tetramethyl-1,3-divinyl disiloxane, however, the vinyl groups can hardly be introduced into the molecular chain terminals due to the very specific polymerization characteristics of the cyclic oligomer. Therefore, vinyl groups to serve as the crosslinking points are usually introduced into the molecules of the fluorine-substituted organopolysiloxane gum by the copolymerization of 1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane and 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane.
A problem in the thus prepared fluorine-substituted organopolysiloxane gum as the principal ingredient of a fluorosilicone rubber composition is the relatively low velocity of the crosslinkforming reaction of the vinyl groups with the free radicals produced from an organic peroxide or of the vinyl groups by the hydrosilation reaction with silicon-bonded hydrogen atoms in an organohydrogenpolysiloxane admixed as a crosslinking agent in the presence of a platinum catalyst presumably due to the three-dimensional bulkiness of the trifluoropropyl groups. Another problem in the conventional fluorosilicone rubbers is the relatively low mechanical strengths thereof in comparison with ordinary organic rubbers including fluorocarbon rubbers.
The reason for the above mentioned defects in the conventional fluorosilicone rubbers is presumably that, when the fluorinated alkyl groups having bulkiness are present in the vicinity of the silicon-bonded vinyl groups, which should provide the crosslinking points, the vinyl groups can pertain to the crosslinking reaction with the free radicals produced from an organic peroxide admixed as a curing agent or silicon-bonded hydrogen atoms with which the vinyl groups enter the hydrosilation reaction to form crosslinks only with a greratly decreased velocity of the crosslinking reaction consequently not to impart the cured silicone rubber composition with full mechanical strengths.
Several attempts and proposals have of course been made in the prior art to solve the above mentioned problem. For example, U.S. Pat. Nos. 4,032,502 and 4,029,629 teach that the curing velocity of a fluorosilicone rubber composition can be controlled by using an organohydrogenpolysiloxane of which the siliconbonded hydrogen atoms are introduced into the polysiloxane molecules in the form of a monofunctional siloxane units of the formula HR.sub.2 SiO.sub.1/2, R being a monovalent hydrocarbon group. Needless to say, the applicability of this method is limited to the compositions curable by the mechanism of hydrosilation and no solution of the problem is provided for the fluorosilicone rubber compositions of the type curable with an organic peroxide as a curing agent.